1. Field of the Invention
The present invention relates to a tracking control apparatus for retrieving a particular selected information track out of a large number of information tracks disposed on a recording carrier and for recording signals onto the recording carrier or reproducing signals recorded on the recording carrier while performing tracking control so that the reproducing position of reproducing means may be located on the information track thus retrieved.
2. Description of the Related Art
A prior art apparatus, employs an optical reproducing apparatus for focusing and radiating an optical beam generated from a light source such as a semiconductor laser onto a disk shaped recording carrier rotated at a predetermined number of revolutions to thereby record signals onto the recording carrier or reproduce signals recorded on the recording carrier.
On this recording carrier, minute tracks each having a width of 0.6 micrometer and a pitch of 1.6 micrometer are disposed in a spiral form or in a concentrical form. When signals are recorded onto the recording carrier or signals recorded on the recording carrier are reproduced, tracking control is performed concurrently therewith so that an optical beam may be always located on a track. An error signal of this tracking control, i.e., a track slippage or slip off signal representing position deviation on the recording carrier between the optical beam and the track is derived by receiving, in an optical detector, reflected light or transmitted light from the recording carrier.
As an actuator for performing tracking control, there is known an actuator comprising a first actuator for moving a focusing lens to move the optical beam on the recording carrier in a track width direction and a second actuator for moving the first actuator in a radial direction of the recording carrier (as described in JP-A-57-147168, for example).
As the first actuator, an actuator so configured as to move the optical beam on the recording carrier in the track width direction by rotating a reflecting mirror is also known (as described in JP-A-56-153562, for example).
In the above described configuration using two actuators, a track deviation signal is applied to the first and second actuators, and tracking control is so performed that the optical beam on the recording carrier may be always located on a track. As for the relationship between the first and second actuators, the first actuator primarily moves for high speed track deviation and the second actuator primarily moves for low speed track deviation.
A large number of tracks are disposed on the recording carrier. Retrieving means is indispensable for retrieving a track having desired information recorded thereon.
Retrieval of a desired track is performed on the basis of addresses given to respective tracks on the recording carrier. The difference between the address of the track now being reproduced and the address of the desired track is derived, and tracking control is stopped. The second actuator is so driven that the optical beam on the recording carrier may cross as many tracks as correspond to the address difference. Thereafter, tracking control is activated again to retrieve the desired track (as described in JP-A-54-92155, for example).
In tracking control in the above described configuration comprising two actuators, movement of the optical beam on the recording carrier caused by the first actuator also moves an optical beam pattern on the optical detector for signal detection. Upon movement of the optical beam pattern on the optical detector, the track deviation signal is influenced, and the value of the track deviation signal with respect to the reference track position changes. When tracking control is activated, therefore, there occurs such a phenomenon that the optical beam on the recording carrier is not actually located on the center of a track even if track deviation appears to be absent judging from the track deviation signal. Further, movement of the focusing lens by the first actuator causes a swing of the optical beam radiated onto the recording carrier. Therefore, the quantity of radiated light is changed. As a result, the amplitude of the reproduced signal is changed or the quality of the recorded signal is degraded. In order to prevent this, the response performance of the second actuator must be improved.
On the other hand, the track deviation signal becomes a sinusoidal signal when the optical beam on the recording carrier crosses a track. However, it is very difficult to obtain a track deviation signal having a perfectly symmetric waveform. When the track deviation signal is asymmetric, crossing a track generates a DC-like component. If the response performance of the second actuator is enhanced and tracking control is not pulled in, the DC-like component of the track deviation signal is amplified and applied to the second actuator, and the optical beam on the recording carrier may slip off in a direction advancing from an outer circumference to an inner circumference or in its opposite direction. This phenomenon occurs when the tracking control is not pulled in at the time of retrieval, or when jumping out of a track is caused by a shock applied from the outside during activation of the tracking control. Occurrence of this phenomenon causes not only a long retrieval time but also a retrieval error, a reproduction error or a recording error, resulting in significantly lowered reliability of the apparatus.
Further, a configuration comprising a single actuator capable of performing the cracking control and the retrieval of a desired track and having a wide movable range is also known (as described in JP-A-60-239943, for example). In case of configuration in which the tracking control is performed by a single actuator, very high control precision of tracking control represented by a value not larger than 0.1 micrometer is required. At the rotation frequency of the recording carrier, therefore, the loop gain of the tracking control system must have a very high value close to 65 dB. If the tracking control is not pulled in when the loop gain of the tracking control is made high, a DC-like component of the track deviation signal is amplified and applied to an actuator, and slip-off similar to that described above occurs.
In case the dynamic range of a circuit for controlling and driving an actuator is asymmetric as well, a DC-like component causing slip-off is generated. That is to say, the actuator moves in a direction advancing from an inner circumference of the recording carrier to an outer circumference thereof in accordance with the eccentricity of tracks. If the maximum signal range of a circuit for moving the actuator in a direction advancing from an inner circumference to an outer circumference is different from the maximum signal range of a circuit for moving the actuator in a direction advancing from an outer circumference to an inner circumference, the signal is saturated and a DC-like component is generated when a track is crossed because the loop gain of the tracking control is very high. This DC-like component also causes a similar phenomenon.